When a borehole drilled for exploration and development of oil and gas is abandoned, a number of cement plugs, in the form of cement slurry, are placed at different depths as required by the regulatory authorities. The plugs are placed so that the fluids from the deeper horizons do not contaminate the water table and upper fresh water sands which may be used as a future water zone.
References to past procedures for inserting plugs in abandoned wells and to installing or testing cement within boreholes may be found in the following list of previously published patents:
1. U.S. Pat. No. 2,171,840--Armentrout and Gudie--describes a method of locating a cement slurry in a well by measuring electrical resistivity, inductance or dielectric capacitance. PA0 2. U.S. Pat. No. 2,220,205--Buckley--describes a method of locating a cement slurry in a well by measuring gamma ray emissions. PA0 3. U.S. Pat. No. 3,489,219--Higgins--describes a method of locating a cement slurry in a well by measuring gamma ray emissions pH, temperature, electromagnetic transmittance, electrical resistivity, dielectric capacitance or magnetic permeability of the cement. PA0 4. U.S. Pat. No. 3,490,528--McClain--describes a method of testing the effectiveness of a cement plug in a borehole by "tagging". PA0 5. Can No. 1,016,065--Campbell--describes a method of locating a cement slurry in a well by measuring radioactive emissions. PA0 6. Can No. 1,094,799--Bennett--describes a method of locating a cement plug in a well by detecting temperature and radioactive emissions. PA0 (1) measuring, when said slurry is in a relatively pure, uncontaminated condition, the density of the fluid cement slurry; PA0 (2) measuring the density of said slurry after it has been inserted down a borehole to a predetermined position, and after said slurry may have been exposed to a potentially contaminated substance; and PA0 (3) comparing the density of the cement slurry as measured when in situ within the borehole with the density of said slurry when said slurry is in a relatively pure, uncontaminated condition and thereby determining the presence of a contaminating substance within said slurry.
According to the procedures typically followed, a cement plug, generally 10 to 50 meters in length, is positioned between zones requiring isolation. This cement plug is positioned in the borehole utilizing the rig's tubular string, and pumping a measured quantity of cement slurry from the surface to the desired depth. This cement slurry displaces the drilling mud at the desired well bore depth and the drill string is withdrawn while the cement hardens.
Generally, two techniques are currently used to determine if the position of the cement plug is correct. One technique uses the rig's tubular string to go down to plug depth to "feel" for the plug, by the procedure known as "tagging". Once the cement has hardened, the tubular string is lowered to the plug top and the surface indication of tubing string weight reduction indicates if the plug has been contacted. The length of pipe is then used to indicate the plug's position.
The second technique is more involved and utilizes radioactive material mixed in the cement at surface, prior to the cement being pumped. The slurry is then pumped and positioned normally at the bottom of the pipe string. Then a measuring device, usually a scintillation gamma ray detector type, is lowered on a wireline to the top of the cement plug. An increase in the gamma ray reading indicates the plug position, and this increase and depth is permanently recorded. With this technique, wireline length is measured and indicates the plug's relative position in the borehole. Both techniques only measure the "position" of the plug.
While previous efforts have been focused on techniques for locating the position of the cement plug being installed in a borehole, or testing the quality of the cement bond between pipe casings and adjacent formations, this invention relates to the testing of the quality, purity or degree of contamination of the cement forming the plug itself.
According to existing procedures, the cement slurry is introduced into the borehole by injecting it into the stream of drilling mud which is being pumped down the pipe string to the bottom of the pipe stem. Ideally, the cement slurry should be introduced in place of the drilling mud for a controlled period of time and then the flow of drilling mud should resume with no mixing between the respective fluids. In actual fact, the temporary substitution of a flow of fluid cement for a portion of the flow of drilling mud within the drill string is not achieved without some degree of mixing occurring between these two fluids, particularly by the time that the cement slurry reaches the bottom of the pipe string.
Additionally, where the borehole is uncased or the casing is perforated, water, rock, gas, and other contaminating materials may enter from the geological formation surrounding the borehole and mix with the cement slurry as it is being transported for deposition at the end of the drill string.
Whenever drilling mud, water, or other contaminants become mixed with fluid cement, the resulting mixture can no longer be said to comprise `pure` cement. As the degree of impurity is increased, eventually the cement will not produce the same quality of plug as would be produced by pure cement. Even if the contaminated mixture should be capable of setting, it will still be of lower strength and may be susceptible to forming cracks and fissures. Once significant cracks or fissures form in a borehole cement plug, the plug is less likely to serve its function of isolating flow of underground fluids between adjacent strata. Over time, limited flow of fluids can enlargen the fissures and result in a complete failure by the plug to serve its intended purpose: to isolate adjacent strata.